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Calcium chloride exists as dihydrate (CaCl2x2h20)

  1. Sep 6, 2004 #1
    Calcium chloride exists as dihydrate (CaCl2x2h20). it has 2 valence.

    according to the definition of valence: "valence, combining capacity of an atom expressed as the number of single bonds the atom can form or the number of electrons an element gives up or accepts when reacting to form a compound. Atoms are called monovalent, divalent, trivalent, or tetravalent, according to whether they form one, two, three, or four bonds."

    this means that the valence of calcium alone (ca2+) is 2 and chloride (cl-) alone is 1. then why the valence of CaCl2 is 2?

    hope for replies!!!
    thanks!
     
  2. jcsd
  3. Sep 6, 2004 #2

    chem_tr

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    Hello,

    You seem to be somewhat confused. If CaCl2.2H2O is a valid chemical formula, then calcium atom has 2+ valence whereas chloride anion has 1-. This is due to the need of valence equilibrium; all plus signs have to find an appropriate minus sign. So calcium forms two bonds with chloride atoms, with one bonds each. Since chlorine is an electronegative atom, it further wants to commit some bonding; it catches two water molecules from the environment and makes hydrogen bonds with them, one each again.

    Hope this helps
    chem_tr
     
  4. Sep 6, 2004 #3
    hi!

    thanks for your reply, but this is not the answer i am looking for. what i wonder is why CaCl2 has a total valence of 2, since Ca2+ alone gives a valence of 2 and Cl- alone has a valence of 1. how come that CaCl2 has a valence of 2 and not 1?

    hope for replies!
    thanks again!
     
  5. Sep 6, 2004 #4

    Gokul43201

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    It is quite wrong to say that CaCl2 has a total valency of 2. It does not. Where did you come across this assertion ?
     
  6. Sep 6, 2004 #5

    chem_tr

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    Well, calcium atom has twenty protons. If you want the neutral atom, calcium has twenty electrons, too. We may write its electronic configuration like that:

    1s2 2s2 2p6 3s2 3p6 4s2

    It expected that alkaline earth cations like calcium have s2 configuration at the end. So, the outer shell has two protons and it is very easy to give them; just find and look at the ionization constants.

    Ca2+: 1s2 2s2 2p6 3s2 3p6

    With this configuration, calcium atom gains the inertness and stability of the corresponding inert gas, argon.

    On the other hand, chlorine has to take one proton to reach the inert gas configuration:

    17Cl: 1s2 2s2 2p6 3s2 3p5

    17Cl-: 1s2 2s2 2p6 3s2 3p6

    Hope this helps
    chem_tr
     
  7. Sep 7, 2004 #6

    ShawnD

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    The valence of CaCl2 isn't 2 because the H2O isn't actually bonded to it. The H2O just happens to be there, and it just happens to be mildly attracted to the CaCl2.
    Think of the scenario like a bunch of magnets. The Ca and Cl2 are like strong magnets close to each other; pulling on one of them will move both. CaCl2 and H2O are like two weak magnets near each other; there is minor attraction between them, but pulling one of them will not move both.
     
  8. Sep 7, 2004 #7


    hello, this is the equation:
    http://www.globalrph.com/osmolarity_calculations.htm

    Calcium chloride: Exists as dihydrate (CaCl2.2h20). Total MW= (40.1 + 35.5 + 35.5+ (2x2) + (2x16)= 146.1 -- Calculations: (1gram CaCl2/146.1) x 1000 x valence (2)= 13.68 meq (vial states: 13.6). Osmolarity: 1 gram/10ml or (100g/liter)/146.1 x 1000 = 684.46 mmol x #species (3) = 2053 mOsm/L (vial states: 2040 mOsm/L)

    hope for replies!

    thank you all!
     
  9. Sep 7, 2004 #8

    Gokul43201

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    In this place they use the term 'valence' very loosely. By valence, they mean, the number of electrons exchanged per mole, in the given reaction.

    In this case, the solvation of CaCl2 involves the loss of 2 electrons by Calcium, and the gain of these 2 electrons by the Chlorines. So, the equivalent weight = molar mass/2 (=molar mass per mole of electrons).

    However, the equivalent weight (molar weight / no. of electrons transfered per molrcule) in general, is not necessarily a constant for a given compound, and is determined from the specific reaction.
     
  10. Sep 7, 2004 #9
    That´s right, the valence of CaCl2 is not two. Simply, the available form of CaCl2 solid samples are in a dihydrate ionic network, so you´ll have to know that not all the mass of the sample is from CaCl2, part of it is water. So if you´re working with that sample you have to calculate the "real" amount of CaCl2 in it.
     
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